The present invention generally relates to carbon fiber and carbon matrix composites such as Carbonxe2x80x94Carbon (hereinafter xe2x80x9cCxe2x80x94Cxe2x80x9d) composites.
It is well known that carbon fiber and Cxe2x80x94C composites each possess a combination of high strength, high fracture toughness, low density, very high thermal conductivity and high electrical conductivity. It is also known that the mechanical strength of carbon fiber and Cxe2x80x94C composites will actually increase as the operating temperature increases, in sharp contrast to most metals and metallic alloys that become softer and weaker as the temperature increases. This combination of attributes would seem to make carbon fiber and/or Cxe2x80x94C composites good candidates for many high temperature applications such as components used in aerospace heat exchangers and aircraft brake pads.
However, the carbon in carbon fiber and Cxe2x80x94C composites tends to oxidize when exposed to air or other oxidizing environments when the temperature exceeds approximately 300xc2x0 C. When the carbon oxidizes, it loses mass with the formation of CO2 and CO gases as oxidation products. This loss in mass directly leads to loss of mechanical strength, as well as loss of integrity, functionality and ultimately to the failure of the component.
In order to protect the Cxe2x80x94C component from oxidizing when subjected to repeated or sustained high temperatures, various barrier coatings may be applied to the components. Known barrier coatings tend to develop micro-cracks over time. These micro-cracks allow oxidizing agents to penetrate the coating and reach the underlying Cxe2x80x94C composite, resulting in loss of mass and ultimately in component failure.
It has been found that the problem of protecting Cxe2x80x94C components from oxidizing when subjected to operating temperatures up to about 1100xc2x0 C. is particularly troublesome when the carbon component is as thin as 3 to 30 mil gauge, and/or complex in shape. Such components may range from fine-dimensioned corrugated fins to complex heat exchanger core assemblies.
There clearly exists a need to prevent oxidation of the carbon fiber or Cxe2x80x94C component over its life cycle.
This need is met by the coatings and methods of application carried out in accordance with the present invention. A fluidized-glass type mixture that is maintained as a liquid precursor at a temperature of between approximately 20-90xc2x0 C. is applied to a component formed of carbon fiber or Cxe2x80x94C composite. Once coated with the precursor, the coated Cxe2x80x94C component is heat-treated or annealed for at least one cycle through a series of gradual heating and cooling steps. This creates a glass coating having a thickness in the range of about 1-10 mils. The thickness may be controlled by the composition of the fluidized glass precursor mixture, the number of application cycles and the annealing parameters.
In accordance with a further aspect of the present invention, a thermally matched refractory coating in the form of a ceramic or glass-ceramic mixture is applied to a Cxe2x80x94C or carbon graphite component by chemical vapor deposition or by plasma-enhanced chemical vapor deposition (PECVD) and infiltration. The specific properties of the refractory coating depend on the deposition conditions, including temperature, plasma power, and partial pressure of the precursor. The refractory coating applied either by CVD or PECVD may serve as the protective coating or be augmented by a fluidized glass coating applied to the coated Cxe2x80x94C component as above stated.